1. Field of the Invention
The present invention relates to a biocular image display apparatus. More particularly, the present invention relates to a head-mounted image display apparatus having the function of tilting a pair of optical systems, together with a pair of image display devices associated therewith, respectively, as one unit in order to match the diopter and the vergence angle with each other. The apparatus is designed to enable an observer to make observation easily without feeling incongruous even at a wide field angle.
2. Discussion of Related Art
One example of conventional biocular image display apparatuses, represented by head-mounted image display apparatuses, will be described with reference to FIG. 10 of the accompanying drawings. This apparatus has a pair of left and right image display units 1L and 1R and a pair of optical systems 2L and 2R associated therewith, respectively. An observer views enlarged virtual images 4L and 4R of the image display surfaces of the image display units 1L and 1R with his/her left and right eyes E.sub.L and E.sub.R. Even if the positions of the left and right virtual images 4L and 4R are not geometrically coincident with each other, the observer can see (fuse) the two images 4L and 4R as a single image by making them coincident with each other in his/her brain. In the example shown in FIG. 10, however, the optical systems 2L and 2R, together with the image display units 1L and 1R, are tilted inwardly toward each other so that the central portions of the virtual images 4L and 4R of the left and right image display surfaces are approximately coincident with each other geometrically.
In general, when an observer looks at an object in daily life, his/her left and right visual lines (i.e. lines of sight) 5L and 5R intersect at the fixation point (this will hereinafter be referred to as "vergence"), and thus the eyes E.sub.L and E.sub.R are focused on the fixation point. When the observer shifts the fixation point further away from or nearer to him or her, the vergence condition of the visual lines 5L and 5R and the focusing of the eyes E.sub.L and E.sub.R change in accordance with the shift of the fixation point.
When the observer gazes at a certain point on the image field of the biocular image display apparatus, the observer's eyes E.sub.L and E.sub.R are focused on the respective positions of the virtual images of the left and right image display surfaces. Consequently, the observer's visual lines 5L and 5R are directed to the corresponding points on the virtual images 4L and 4R of the left and right image display surfaces. In the example shown in FIG. 10, the apparatus is arranged such that the central portions of the virtual images 4L and 4R of the left and right image display surfaces are approximately coincident with each other geometrically. Therefore, in an area of small field angle (i.e. the hatched portion in the figure), the visual lines 5L and 5R intersect (converge) substantially on the virtual images 4L and 4R. Accordingly, the condition of observation is close to that in daily life.
However, in an area of large field angle (i.e. the area on each side of the hatched portion in the figure), the displacement between the corresponding image points becomes large. The visual lines 5L' and 5R' formed when the observer views an edge of each image display surface converge considerably nearer to him or her than in a case where the observer looks at the center of each image display surface. Consequently, it appears to the observer that both of the edges of the image field lie undesirably close to him or her as shown by the curve 6 in FIG. 10. If the imbalance between the focusing and vergence of the eyes E.sub.L and E.sub.R increases, the observer feels discomfort or incongruity. Moreover, it becomes difficult to fuse the left and right images. In such a case, the image to be observed may look double or appear to be flickering.
When the observer looks obliquely upwards as shown in FIG. 11, the left and right visual lines 5L" and 5R" do not intersect but are vertically displaced relative to each other. As the vertical displacement increases, it becomes difficult to fuse the left and right images, and the observer feels difficulty in viewing. When the observer views only the displayed image, he or she can hardly notice the displacement as long as it is small. However, when the apparatus is used in the superimpose mode, in which the image for observation is superimposed on an external scene, or in the see-around mode, in which an external scene is seen in the immediate neighborhood of the observation image, it is likely that either the observation image or the external scene will become difficult to fuse.